SciCombinator

Discover the most talked about and latest scientific content & concepts.

Journal: The journal of physical chemistry letters

23

Despite many recent developments in designing and screening catalysts for improved performance, transition-metal oxides continue to prove challenging due to the myriad inherent complexities of the systems and the possible morphologies that it can exhibit. Herein we propose a structural descriptor, the adjusted coordination number (ACN), which can generalize the reactivity of the oxygen sites over the many possible surface facets and defects of a transition-metal oxide. We demonstrate the strong correlation of this geometric descriptor with the electronic and energetic properties of the active sites on five facets of four transition-metal oxides. We then use the structural descriptor to predict C-H activation energies, to show the great potential of using ACN for the prediction of catalytic performance. This study presents a first look into quantifying the relation between active site structure and reactivity of transition-metal-oxide catalysts.

Concepts: Oxygen, Metabolism, Catalysis, Nitrogen, Oxide, Transition metals, Transition metal oxides

13

The surface tension of water is an important parameter for many biological or industrial processes, and roughly a factor of three higher than that of non-polar liquids such as oils, which is usually attributed to hydrogen bonding and dipolar interactions. Here we show by studying the formation of water drops that the surface tension of a freshly created water surface is even higher (~ 90 mNm(-1)) than under equilibrium conditions (~ 72 mNm(-1)) with a relaxation process occurring on a long timescale (~ 1 ms). Dynamic adsorption effects of protons or hydroxides may be at the origin of this dynamic surface tension. However, changing the pH does not significantly change the dynamic surface tension. It also seems unlikely that hydrogen bonding or dipole orientation effects play any role at the relatively long time scale probed in the experiments.

Concepts: Oxygen, Water, Hydrogen, Atom, Hydrogen bond, Liquid, Surface tension, Hydronium

5

Super-resolution microscopy typically achieves high spatial resolution, but the temporal resolution remains low. We report super temporal-resolved microscopy (STReM) to improve the temporal resolution of 2D super-resolution microscopy by a factor of 20 compared to that of the traditional camera-limited frame rate. This is achieved by rotating a phase mask in the Fourier plane during data acquisition and then recovering the temporal information by fitting the point spread function (PSF) orientations. The feasibility of this technique is verified with both simulated and experimental 2D adsorption/desorption and 2D emitter transport. When STReM is applied to measure protein adsorption at a glass surface, previously unseen dynamics are revealed.

Concepts: Function, Optics, Microscopy, The Point, Frame rate, Point spread function, Spread betting, Astronomical seeing

5

Basing on ab initio density functional calculations, we performed a comprehensive investigation of the general graphitization tendency in rocksalt-type structures. In this paper, we determine the critical slab thickness for a range of ionic cubic crystal systems, below which a spontaneous conversion from a cubic to a layered graphitic-like structure occurs. This conversion is driven by surface energy reduction. Using only fundamental parameters of the compounds such as the Allen electronegativity and ionic radius of the metal atom, we also develop an analytical relation to estimate the critical number of layers.

Concepts: Crystal, Crystallography, Chemical bond, Carbon, Caesium, Crystal system, Cubic crystal system, Space group

5

Simultaneously accurate and efficient prediction of molecular properties throughout chemical compound space is a critical ingredient toward rational compound design in chemical and pharmaceutical industries. Aiming toward this goal, we develop and apply a systematic hierarchy of efficient empirical methods to estimate atomization and total energies of molecules. These methods range from a simple sum over atoms, to addition of bond energies, to pairwise interatomic force fields, reaching to the more sophisticated machine learning approaches that are capable of describing collective interactions between many atoms or bonds. In the case of equilibrium molecular geometries, even simple pairwise force fields demonstrate prediction accuracy comparable to benchmark energies calculated using density functional theory with hybrid exchange-correlation functionals; however, accounting for the collective many-body interactions proves to be essential for approaching the “holy grail” of chemical accuracy of 1 kcal/mol for both equilibrium and out-of-equilibrium geometries. This remarkable accuracy is achieved by a vectorized representation of molecules (so-called Bag of Bonds model) that exhibits strong nonlocality in chemical space. In addition, the same representation allows us to predict accurate electronic properties of molecules, such as their polarizability and molecular frontier orbital energies.

Concepts: Scientific method, Molecule, Chemistry, Chemical bond, Chemical substance, Computational chemistry, Density functional theory, Chemical compound

4

The lack of molecular mechanistic understanding of the interaction between metal complexes and biomolecules hampers their potential medical use. Herein we present a robust procedure combining resonant X-ray emission spectroscopy and multiscale molecular dynamics simulations, which allows for straightforward elucidation of the precise interaction mechanism at the atomic level. The report unveils an unforeseen hydrolysis process and DNA binding of [Pt{N(p-HC6F4)CH2}2py2] (Pt103), which showed potential cytotoxic activity in the past. Pt103 preferentially coordinates to adjacent adenine sites, instead of guanine sites as in cisplatin, because of its hydrogen bond ability. Comparison with previous research on cisplatin suggests that selective binding to guanine or adenine may be achieved by controlling the acidity of the compound.

Concepts: DNA, Protein, Oxygen, Pharmacology, Hydrogen, Molecule, Atom, Biochemistry

4

Dissipative forces are ubiquitous and thus constitute an essential part of realistic physical theories. However, quantization of dissipation has remained an open challenge for nearly a century. We construct a quantum counterpart of classical friction, a velocity-dependent force acting against the direction of motion. In particular, a translationary invariant Lindblad equation is derived satisfying the appropriate dynamical relations for the coordinate and momentum (i.e., the Ehrenfest equations). Numerical simulations establish that the model approximately equilibrates. These findings significantly advance a long search for a universally valid Lindblad model of quantum friction and open opportunities for exploring novel dissipation phenomena.

Concepts: Quantum mechanics, Physics, General relativity, Mass, Force, Classical mechanics, Friction, Equations

4

Molecular photon upconversion via triplet-triplet annihilation (TTA-UC), combining two or more low energy photons to generate a higher energy excited state, is an intriguing strategy to surpass the maximum efficiency for a single junction solar cell (<34%). Here, we introduce self-assembled bilayers on metal oxide surfaces as a strategy to facilitate TTA-UC emission and demonstrate direct charge separation of the upconverted state. A 3-fold enhancement in transient photocurrent is achieved at light intensities as low as two equivalent suns. This strategy is simple, modular and offers unprecedented geometric and spatial control of the donor-acceptor interactions at an interface. These results are a key stepping stone toward the realization of an efficient TTA-UC solar cell that can circumvent the Shockley-Queisser limit.

Concepts: Electron, Photon, Energy, Quantum mechanics, Light, Photoelectric effect, Stimulated emission, Atom

3

Nitrides, carbides and borides of transition metals are an attractive class of hard materials. Our recent preliminary explorations of the binary chemical compounds indicated that chromium-based materials are among the hardest transition metal compounds. Motivated by this, here we explore in detail the binary Cr-B, Cr-C and Cr-N systems using global optimization techniques. Calculated enthalpy of formation and hardness of predicted materials were used for Pareto optimization to define the hardest materials with lowest energy. Our calculations recover all numerous known stable compounds (except Cr23C6 with its large unit cell) and discover a novel stable phase Pmn21-Cr2C. We resolve the structure of Cr2N and find it to be of anti-CaCl2 type (space group Pnnm). Many of these phases possess remarkable hardness, but only CrB4 is superhard (Vickers hardness 48 GPa). Among chromium compounds, borides generally possess highest hardnesses and greatest stability. Under pressure, we predict stabilization of a TMDC-like phase of Cr2N, a WC-type phase of CrN, and a new compound CrN4. Nitrogen-rich chromium nitride CrN4 is a high energy-density material featuring polymeric nitrogen chains. In the presence of metal atoms (e.g. Cr) polymerization of nitrogen takes place at much lower pressures: CrN4 becomes stable at ~15 GPa (cf. 110 GPa for synthesis of pure polymeric nitrogen).

Concepts: Iron, Chemical substance, Metal, Chemical compound, Transition metal, Osmium, Chromium, Hardness

2

Solid polymer electrolytes (SPE) have the potential to increase both the energy density and stability of lithium-based batteries, but low Li+ conductivity remains a barrier to technological viability. SPEs are designed to maximize Li+ diffusivity relative to the anion, while maintaining sufficient salt solubility. It is thus remarkable that polyethylene oxide (PEO), the most widely used SPE, exhibits Li+ diffusivity that is an order of magnitude smaller than that of typical counter-ions at moderate salt concentrations. We show that Lewis-basic polymers like PEO favor slow cation and rapid anion diffusion while this relationship can be reversed in Lewis-acidic polymers. Using molecular dynamics, polyboranes are identified that achieve up to ten-fold increases in Li+ diffusivities and signicant decreases in anion diffusivities, relative to PEO in the dilute-ion regime. These results illustrate a general principle for increasing Li+ diffusivity and transference number with chemistries that exhibit weaker cation and stronger anion coordination.

Concepts: Water, Molecule, Chemistry, Atom, Sodium chloride, Ion, Physical chemistry, Potassium